[Bug inline-asm/82001] New: [5/6/7/8 regression] wrong code when two functions differ only in inline asm register constraints

["peter at cordes dot ca" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82001

            Bug ID: 82001
           Summary: [5/6/7/8 regression] wrong code when two functions
                    differ only in inline asm register constraints
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: inline-asm
          Assignee: unassigned at gcc dot gnu.org
          Reporter: peter at cordes dot ca
  Target Milestone: ---
            Target: x86_64-*-*, i?86-*-*

When a single compilation unit contains two functions that are identical other
than specific-register constraints in an asm statement, they are incorrectly
treated as exactly identical and the same code is emitted for both.

This happens at -O2 or higher, not -O1.

I was able to construct this test-case with two functions that are both
plausibly useful.  (Although this actually came up while discussing a beginner
SO question about inline asm. 
https://stackoverflow.com/questions/45910530/how-to-write-a-short-block-of-inline-gnu-extended-assembly-to-swap-the-values-of#comment78780443_45910796)

int mullo(int a, int b) {
    asm("mul %%edx   # %%1 was %1" : "+a" (a), "+d" (b));
    return a;
}

int mulhi(int a, int b) {
    asm("mul %%edx   # %%1 was %1" : "+d" (a), "+a" (b));
    return a;
}


gcc8.0.0-snapshot 20170827 -O3 (https://godbolt.org/g/CYjnGg) compiles them
both to

        movl    %edi, %eax      # a, a
        movl    %esi, %edx      # b, b
        mul %edx   # %1 was %edx        # b
        ret

Any difference in the asm string, or in the clobber registers, makes them not
match.  Also, an "m" constraint is seen as different from an "r" or
specific-register constraint, but "imr" can "match" an "r"

In gcc6/7/8, both functions use the correct asm for the 1st function.  Swapping
the order changes the asm to the other one.

In gcc5.x, both functions use the correct asm for the 2nd function.

In gcc4.9.4, both functions are compiled correctly.